This invention relates generally to a ballast for starting a fluorescent lamp, and more particularly to an ignition scheme for igniting a fluorescent lamp following a predetermined period of time during which the lamp filaments are preheated.
Fluorescent lamps often require that their filaments (electrodes) be preheated prior to lamp ignition. Preheating is often controlled by an automatic switch commonly referred to as a starter. Once the filaments have been preheated, one or more large voltage pulses, generated by an ignitor, are applied to the fluorescent lamp filaments. These voltage pulses are commonly referred to as ignition pulses. Filaments are also frequently heated following ignition.
A starter typically includes two electrodes, at least one of which is bimetallic, enclosed in an argon/helium glass bulb. When the mains are switched on, a glow discharge starts between the electrodes. The heat from the glow discharge causes the bimetallic electrodes to bend toward and come into contact with each other. Current now flows through the electrodes and lamp filaments heating the latter. In the meanwhile, the glow discharge within the starter bulb has stopped allowing the electrodes to cool down and thereby separate from each other. When the electrodes separate, the relatively heavy current flowing through the inductive ballast and lamp filaments is interrupted. Voltage pulses in the order of 1000 volts, sufficient for lamp ignition, are now applied across the lamp filaments. In the event that the lamp does not ignite, the filaments are once again preheated prior to attempting to ignite the lamp.
The amount of heat applied to the lamp filaments prior to application of the ignition pulses thereto is critical in starting the lamp. In lamps that require preheating, it is difficult to start a lamp when the filaments have not been sufficiently preheated. Sputterring of the filaments during unsuccessful ignition can occur. Blackening of the fluorescent bulb and a decrease in lamp life often follows.
Starters, such as described above, do not sufficiently control the amount of heat being applied to the filaments prior to attempted ignition. More particularly, the amount of heat applied to the filaments varies based, in part, on the amount of time that the bimetallic elements remain in contact with each other. An unreliable ignition scheme with consequential sputtering of filament (emitter) material can result.
Ignition of the lamp also depends on the amount of energy within the high voltage (ignition) pulses generated by the ignitor. The amount of energy delivered to the lamp by the high voltage pulses can vary and depends, in part, on the number of high voltage pulses produced over a predetermined period of time. When an insufficient amount of energy is delivered to the lamp, the lamp will not ignite even though the filaments have been adequately preheated.
Accordingly, it is desirable to provide an improved ballast scheme in which the fluorescent lamp filaments are sufficiently preheated prior to turning on the ignitor. The ballast should, in particular, more precisely control the length of time that the filaments are preheated prior to attempted lamp ignition. Circuitry employed for controlling the length of time that the filaments are preheated should be isolated from the ignitor to protect the former from the high voltage levels generated by the latter. The improved ballast scheme also should generate ignition pulses having an overall higher energy level than generated by a conventional ignitor so as to provide a more reliable ignition scheme.